Emergence of multiple Diaporthe species causing kiwifruit rot and occurrence of resistance to a methyl benzimidazole carbamate fungicide in South Korea

Abstract

In recent years, new fungal diseases in kiwifruits have emerged worldwide. This study is the first report of a postharvest kiwifruit rot in South Korea caused by Diaporthe eres and related fungal pathogens. After single spore isolation of Diaporthe-like fungi isolated from rotten kiwifruits, isolates were identified based on the multi-locus phylogeny using the nuclear rDNA internal transcribed spacer and three nuclear protein-coding loci. Among the 41 single-spore isolates, a majority of the isolates (75%) were identified as D. eres, followed by D. unshiuensis, D. phaseolorum, D. cf. pseudomangiferae and D. caryae. We evaluated fungicide resistance of the 41 isolates and confirmed that a significant fraction of the isolates (15%) was resistant to thiophanate-methyl, raising a concern on overuse of this type of fungicide in kiwi orchards. By sequencing a region of β-tubulin gene, we identified two point mutations altering amino acid sequences correlated with the observed resistance. For effective diagnosis of incidence of the fungicide resistance in the fields, we developed allele-specific PCR analyses detecting the point mutations that confer reduced sensitivity to thiophanate-methyl. Genetic structure of the 41 isolates suggested that there were at least five independent mutational events for acquiring the fungicide resistance. Unexpectedly, there was no clear distinction in the genetic structure between the 41 kiwifruit isolates and Chinese isolates that caused grapevine dieback disease, suggesting these Diaporthe species are non-host-selective pathogens that could damage different horticultural crops of economic importance. This study will help develop a discerned protection program to cope with the ongoing and upcoming threats of these genetically diverse kiwifruit rot pathogens.